Static random access memory cell

ABSTRACT

A static random access memory (SRAM) cell includes substrate, a first semiconductor fin, a first gate structure, a second semiconductor fin, and a second gate structure. The substrate has a first p-well and an n-well bordering the first p-well. The first semiconductor fin extends within the first p-well. The first gate structure extends across the first semiconductor fin and forms a first write-port pull-down transistor with the first semiconductor fin. The second semiconductor fin extends within the n-well. The second gate structure extends across the second semiconductor fin and forms a first write-port pull-up transistor with the second semiconductor fin. A channel region of the first write-port pull-down transistor has a higher doping concentration than a channel region of the first write-port pull-up transistor.

PRIORITY CLAIM AND CROSS-REFERENCE

The present application is a continuation Application of the U.S.application Ser. No. 16/721,632, filed Dec. 19, 2019, now U.S. Pat. No.11,355,499, issued Jun. 7, 2022, which is a continuation Application ofthe U.S. application Ser. No. 15/354,052, filed Nov. 17, 2016, now U.S.Pat. No. 10,515,969, issued Dec. 24, 2019, all of which are hereinincorporated by reference in their entirety.

BACKGROUND

A current common demand for an electronic circuit and particularly forelectronic circuits manufactured as integrated circuits in semiconductorprocesses is an on-board or embedded array of memory storage elements.These elements may be provided as dynamic random access memory (DRAM)cells and alternatively as static random access memory (SRAM) cells.DRAM and SRAM memories are described as “volatile” memory cells, in thatif the power to the integrated circuit device is removed, the storeddata will be lost. SRAM cells retain data so long as a supply voltage ispresent. Recent system on a chip (SOC) designs often incorporate one ormore “cores”. These cores are often predesigned popular processorsarranged with a level one (L1) cache memory of SRAM cells laid out nearor adjacent to the processor to make very fast processing operationspossible.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a circuit diagram of a semiconductor device 10 in accordancewith various embodiments of the present disclosure.

FIGS. 2A to 8A are top views of a method for manufacturing asemiconductor device at various stages in accordance with someembodiments of the present disclosure.

FIGS. 2B to 8B are cross-sectional views taken along lines A-A and B-Bof FIGS. 2A to 8A.

FIG. 9 is a top view of a semiconductor device in accordance with someembodiments of the present disclosure.

FIGS. 10A to 13A are top views of a method for manufacturing asemiconductor device at various stages in accordance with someembodiments of the present disclosure.

FIGS. 10B to 13B are cross-sectional views taken along lines A-A and B-Bof FIGS. 9 to 13A.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The present disclosure will be described with respect to embodiments, astatic random-access memory (SRAM) formed of fin field effecttransistors (FinFETs). The embodiments of the disclosure may also beapplied, however, to a variety of integrated circuits. Variousembodiments will be explained in detail with reference to theaccompanying drawings.

Static random-access memory (SRAM) is a type of volatile semiconductormemory that uses bistable latching circuitry to store each bit. Each bitin an SRAM is stored on four transistors (PU-1, PU-2, PD-1, and PD-2)that form two cross-coupled inverters. This SRAM cell has two stablestates which are used to denote 0 and 1. Two additional accesstransistors (PG-1 and PG-2) serve to control the access to a storagecell during read and write operations.

FIG. 1 is a circuit diagram of a semiconductor device 10 in accordancewith various embodiments of the present disclosure. In FIG. 1 , thesemiconductor device 10 is an eight transistor static random accessmemory (8T-SRAM) cell. The semiconductor device 10 includes a write port12 and a read port 14. The write port 12 includes a first inverter 102formed by a pull-up transistor PU-I and a pull-down transistor PD-1. Thewrite port 12 further includes a second inverter 104 formed by a pull-uptransistor PU-2 and a pull-down transistor PD-2. Furthermore, both thefirst inverter 102 and second inverter 104 are coupled between a voltagebus Vdd and a ground potential Vss. In some embodiments, the pull-uptransistor PU-1 and PU-2 can be p-type metal oxide semiconductor (PMOS)transistors while the pull-down transistors PD-1 and PD-2 can be n-typemetal oxide semiconductor (NMOS) transistors, and the present disclosureis not limited in this respect.

In FIG. 1 , the first inverter 102 and the second inverter 104 arecross-coupled. That is, the first inverter 102 has an input connected tothe output of the second inverter 104. Likewise, the second inverter 104has an input connected to the output of the first inverter 102. Theoutput of the first inverter 102 is referred to as a storage node 103.Likewise, the output of the second inverter 104 is referred to as astorage node 105. In a normal operating mode, the storage node 103 is inthe opposite logic state as the storage node 105. By employing the twocross-coupled inverters, the semiconductor device 10 can hold the datausing a latched structure so that the stored data will not be lostwithout applying a refresh cycle as long as power is supplied throughVdd.

In FIG. 1 , the write port 12 further includes a first pass-gatetransistor PG-1 connected between a bit line BL and the output 103 ofthe first inverter 102. The write port 12 further includes a secondpass-gate transistor PG-2 connected between a bit line BLB and theoutput 105 of the second inverter 104. The gates of the first pass-gatetransistor PG-1 and the second pass-gate transistor PG-2 are connectedto a word line WWL, which connects semiconductor devices in a row of theSRAM array.

In operation, if the pass-gate transistors PG-1 and PG-2 are inactive,the write port 12 will maintain the complementary values at storagenodes 103 and 105 indefinitely as long as power is provided through Vdd.This is so because each inverter of the pair of cross coupled invertersdrives the input of the other, thereby maintaining the voltages at thestorage nodes. This situation will remain stable until the power isremoved from the semiconductor device 10, or, a write cycle is performedchanging the stored data at the storage nodes.

The read port 14 includes a read port pull down transistor RPD and aread port pass gate transistor RPG. A read word line (RWL) is providedthat is dedicated to “reads” only. The previous word line WWL is a writeonly word line. A separate read port can reduce the possibility of “readdisturbs”, because the data stored in the bit cell affected by the readoperations; instead, the read pull down transistor RPD is either on oroff based on the storage node 103 voltage that is coupled to the gate ofthe transistor RPD. When the read word line RWL has a positive voltageplaced on it, read pass gate transistor RPG turns on and couples theread bit line RBL to the read pull down transistor RPD, and the readport therefore outputs a corresponding data bit on the read bit lineRBL. In many applications, SRAM arrays of many bit cells are used thatstore data or programs for retrieval and use later. The SRAM cells mayexperience many more read operations than write operations in the sametime period. Thus, the read operations can be isolated from the bit cellby the read port circuit 14.

The structure of the semiconductor device 10 in FIG. 1 is described inthe context of the 8T-SRAM. One of ordinary skill in the art, however,should understand that features of the various embodiments describedherein may be used for forming other types of devices, such as a IOT-SRAM memory device, or memory devices other than SRAMs. Furthermore,embodiments of the present disclosure may be used as stand-alone memorydevices, memory devices integrated with other integrated circuitry, orthe like. Accordingly, the embodiments discussed herein are illustrativeof specific ways to make and use the disclosure, and do not limit thescope of the disclosure.

FIGS. 2A to 8A are top views of a method for manufacturing asemiconductor device at various stages in accordance with someembodiments of the present disclosure, and FIGS. 2B to 8B arecross-sectional views taken along lines A-A and B-B of FIGS. 2A to 8A.Reference is made to FIGS. 2A and 2B. A substrate 210 is provided. Thesubstrate 210 has a write portion 210 w and a read portion 210 radjacent to the write portion 210 w. The substrate 210 further includesa plurality of semiconductor fins 212-215 and 218 protruded from thesubstrate 210. The semiconductor fins 212-215 are disposed in the writeportion 210 w of the substrate 210, and the semiconductor fin 218 isdisposed in the read portion 210 r of the substrate 210. Further, thewrite portion 210 w has a heavy doping area H, and a portion of thesemiconductor fin 215 is disposed in the heavy doping area H. Thesemiconductor fins 212-215 and 218 serves as active regions oftransistors (see FIGS. 8A and 13A). It is note that the numbers of thesemiconductor fins 212-215 and 218 in FIG. 2A are illustrative, andshould not limit the claimed scope of the present disclosure. A personhaving ordinary skill in the art may select suitable numbers for thesemiconductor fins 212-215 and 218 according to actual situations. Byway of example and not limitation, the semiconductor fin 215 may bereferred to as a first semiconductor fin, the semiconductor fin 213 maybe referred to as a second semiconductor fin, the semiconductor fin 212may be referred to as a third semiconductor fin, the semiconductor fin218 may be referred to as a fourth semiconductor fin, and thesemiconductor fin 214 may be referred to as a fifth semiconductor fin.

In some embodiments, the substrate 210 includes silicon. Alternatively,the substrate 210 may include germanium, silicon germanium, galliumarsenide or other appropriate semiconductor materials. Alsoalternatively, the substrate 210 may include an epitaxial layer. Forexample, the substrate 210 may have an epitaxial layer overlying a bulksemiconductor. Further, the substrate 210 may be strained forperformance enhancement. For example, the epitaxial layer may include asemiconductor material different from that of the bulk semiconductor,such as a layer of silicon germanium overlying bulk silicon or a layerof silicon overlying bulk silicon germanium. Such strained substrate maybe formed by selective epitaxial growth (SEG). Furthermore, thesubstrate 210 may include a semiconductor-on-insulator (SOI) structure.Also alternatively, the substrate 210 may include a buried dielectriclayer, such as a buried oxide (BOX) layer, such as that formed byseparation by implantation of oxygen (SIMOX) technology, wafer bonding,SEG, or other appropriate method.

At least one first well 202 and at least one second well 204 are formedin the substrate 210. For example, in FIG. 2A, the second well 204 isformed between two of the first wells 202. In some embodiments, thefirst wells 202 are p-well region, and the second well 204 is an n-wellregion, and the claimed scope is not limited in this respect. In someembodiments, the first wells 202 are implanted with P dopant material,such as arsenic ions, and the second well 204 is implanted with N dopantmaterial such as boron ions. During the implantation of the first wells202, the second well 204 is covered with masks (such as photoresist),and during implantation of the second well 204, the first wells 202 arecovered with masks (such as photoresist). [NOTE: please check if thehighlight description is correct.

The semiconductor fins 212, 213, 214, 215, and 218 may be formed, forexample, by patterning and etching the substrate 210 usingphotolithography techniques. In some embodiments, a layer of photoresistmaterial (not shown) is deposited over the substrate 210. The layer ofphotoresist material is irradiated (exposed) in accordance with adesired pattern (the semiconductor fins 212-215 and 218 in this case)and developed to remove a portion of the photoresist material. Theremaining photoresist material protects the underlying material fromsubsequent processing steps, such as etching. It should be noted thatother masks, such as an oxide or silicon nitride mask, may also be usedin the etching process.

In some other embodiments, the semiconductor fins 212-215 and 218 may beepitaxially grown. For example, exposed portions of an underlyingmaterial, such as an exposed portion of the substrate 210, may be usedin an epitaxial process to form the semiconductor fins 212-215 and 218.A mask may be used to control the shape of the semiconductor fins212-215 and 218 during the epitaxial growth process.

Reference is made to FIGS. 3A and 3B. Portions of the semiconductor fins213 and 214 are removed. For example, a photomask (not shown) containingpatterns for both the semiconductor fins 213 and 214 are used to protectportions of the semiconductor fins 213 and 214 to be kept. Exposedportions of both the semiconductor fins 213 and 214 are then etched atthe same time.

Subsequently, a plurality of isolation structures 205 are formed on thesubstrate 210. The isolation structures 205, which act as shallow trenchisolations (STIs) around the semiconductor fins 212-215 and 218, may beformed by chemical vapor deposition (CVD) techniques usingtetra-ethyl-ortho-silicate (TEOS) and oxygen as a precursor. In someother embodiments, the isolation structures 205 may be formed byimplanting ions, such as oxygen, nitrogen, carbon, or the like, into thesubstrate 210. In yet some other embodiments, the isolation structures205 are insulator layers of a SOI wafer.

Reference is made to FIGS. 4A and 4B. A mask 310 is formed on the readport 210 r of the substrate 210. That is, the mask 310 covers thesemiconductor fin 218 while leaves the semiconductor fins 212-215uncovered. Then, a channel doping process is performed to form channeldoping layers 216 respectively in the semiconductor fins 212-215. Insome embodiments, N dopants or P dopants such as arsenic (As) or boron(B) can be doped into the semiconductor fins 212-215 to form the channeldoping layers 216. In some embodiments, the implantation results in asurface layer of the semiconductor fins 212-215 to be doped, wherein thesurface layer includes the top surface layer and the opposite sidewallsurface layers of the semiconductor fins 212-215. The inner portions ofthe semiconductor fins 212-215 wrapped by the channel doping layers 216,however, may be free from the dopants. In alternative embodiments, theimplantation results in an entirety of semiconductor fins 212-215 to bedoped with the dopants.

Reference is made to FIGS. 5A and 5B. The mask 310 of FIGS. 4A and 4B isremoved. In some embodiments, another mask 320 is formed on thesubstrate 210. The mask 320 covers the write portion 210 w and the readportion 210 r except the heavy doping area H. In other words, the mask320 exposes the heavy doping area H. Then, another channel dopingprocess is performed to form a channel doping layer 217 in the portionof the semiconductor fin 215 in the heavy doping area H. In someembodiments, N dopants or P dopants such as arsenic (As) or boron (B)can be doped into the semiconductor fin 215 to form the channel dopinglayer 217. In some embodiments, the implantation results in a surfacelayer of the semiconductor fin 215 to be doped, wherein the surfacelayer includes the top surface layer and the opposite sidewall surfacelayers of the semiconductor fin 215. The inner portions of thesemiconductor fin 215 wrapped by the channel doping layer 217, however,may be free from the dopants. In alternative embodiments, theimplantation results in an entirety of semiconductor fin 215 to be dopedwith dopants. After the process of FIGS. 5A and 5B, the dopingconcentration of the channel doping layer 217 is higher than the dopingconcentration of the channel doping layer 216.

The implantation process of FIGS. 4A-5B is illustrative. In some otherembodiments, the channel doping layers 216 and 217 may be formedseparately. For example, a mask can be formed to cover the read portion210 r and the heavy doping area H, and the exposed semiconductor fins212-215 are doped to respectively form the channel doping layers 216therein. The mask is then removed, and the mask 320 of FIG. 5A is formedon the substrate 210. The portion of the semiconductor fin 215 in theheavy doping area H is then doped to form the channel doping layer 217therein. In some other embodiments, the process of FIGS. 5A and 5B canbe omitted. That is, the semiconductor fins 212-215 respectively includethe channel doping layers 216.

Reference is made to FIGS. 6A and 6B. A plurality of gate dielectrics230 are formed on the semiconductor fins 212-215 and 218. For clarity,the gate dielectrics 230 are illustrated in FIG. 6B and are omitted inFIG. 6A. The gate dielectrics 230, which prevents electron depletion,may include, for example, a high-k dielectric material such as metaloxides, metal nitrides, metal silicates, transition metal-oxides,transition metal-nitrides, transition metal-silicates, oxynitrides ofmetals, metal aluminates, zirconium silicate, zirconium aluminate, orcombinations thereof. Some embodiments may include hafnium oxide (HfO₂),hafnium silicon oxide (HfSiO), hafnium silicon oxynitride (HfSiON),hafnium tantalum oxide (HfTaO), hafnium titanium oxide (HMO), hafniumzirconium oxide (HfZrO), lanthanum oxide (LaO), zirconium oxide (ZrO),titanium oxide (TiO), tantalum oxide (Ta₂O₅), yttrium oxide (Y₂O₃),strontium titanium oxide (SrTiO₃, STO), barium titanium oxide (BaTiO₃,BTO), barium zirconium oxide (BaZrO), hafnium lanthanum oxide (HfLaO),lanthanum silicon oxide (LaSiO), aluminum silicon oxide (AlSiO),aluminum oxide (Al₂O₃), silicon nitride (Si₃N₄), oxynitrides (SiON), andcombinations thereof. The gate dielectrics 230 may have a multilayerstructure such as one layer of silicon oxide (e.g., interfacial layer)and another layer of high-k material. The gate dielectrics 230 may beformed using chemical vapor deposition (CVD), physical vapor deposition(PVD), atomic layer deposition (ALD), thermal oxide, ozone oxidation,other suitable processes, or combinations thereof.

A plurality of dummy gates 241, 243, 245, 247, and 249 are then formedon portions of the semiconductor fins 212-215 and 218 and expose anotherportions of the semiconductor fins 212-215 and 218. The dummy gates 241,243, 245, 247, and 249 are formed over the substrate 210 to cover thegate dielectrics 230 and the portions of the semiconductor fins 212-215and 218. As shown in FIG. 6B, the gate dielectrics 230 are disposedbetween the dummy gates 241, 243, 245, 247, and 249 and thesemiconductor fins 212-215 and 218. In greater detail, the dummy gate241 is formed on portions of the semiconductor fins 212 and 213, andfurther on a portion of the semiconductor fin 214 in some embodiments(as shown in FIG. 6A); the dummy gate 243 is formed on a portion of thesemiconductor fin 215; the dummy gate 245 is formed on a portion of thesemiconductor fin 218, the dummy gate 247 is formed on another portionof the semiconductor fin 212, and the dummy gate 249 is formed onportions of the semiconductor fins 214, 215, and 218, and further onanother portion of the semiconductor fin 213 in some embodiments (asshown in FIG. 6A).

In some embodiments, the dummy gates 241, 243, 245, 247, and 249includes a semiconductor material such as polysilicon, amorphoussilicon, or the like. The dummy gates 241, 243, 245, 247, and 249 may bedeposited doped or undoped. For example, in some embodiments, the dummygates 241, 243, 245, 247, and 249 includes polysilicon deposited undopedby low-pressure chemical vapor deposition (LPCVD). The polysilicon mayalso be deposited, for example, by furnace deposition of an in-situdoped polysilicon.

A plurality of gate spacers (not shown) are formed over the substrate210 and along the sides of the dummy gates 241, 243, 245, 247, and 249.In some embodiments, the gate spacers may include silicon oxide, siliconnitride, silicon oxy-nitride, or other suitable material. The gatespacers may include a single layer or multilayer structure. A blanketlayer of the gate spacers may be formed by CVD, PVD, ALD, or othersuitable technique. Then, an anisotropic etching is performed on theblanket layer to form a pair of the gate spacers on two sides of thedummy gates 241, 243, 245, 247, and 249.

In FIG. 6A, the semiconductor fin 212 and the dummy gate 241 form apull-down transistor PD-1, and the semiconductor fin 213 and the dummygate 241 form a pull-up transistor PU-1. In other words, the pull-downtransistor PD-1 and the pull-up transistor PU-1 share the dummy gate241. The semiconductor fin 215 and the dummy gate 243 form a pass-gatetransistor PG-2. The semiconductor fin 218 and the dummy gate 245 form aread-pass-gate transistor RPG. The semiconductor fin 212 and the dummygate 247 form another pass-gate transistor PG-1. The semiconductor fin214 and the dummy gate 249 form another pull-up transistor PU-2, thesemiconductor fin 215 and the dummy gate 249 form another pull-downtransistor PD-2, and the semiconductor fin 218 and the dummy gate 249form a read-pull-down transistor RPD. In other words, the pull-uptransistor PU-2, the pull-down transistor PD-2 and the read-pull-downtransistor RPD share the dummy gate 249. Therefore, the transistors inthe write portion 210 w (i.e., transistors PD-1, PD-2, PU-1, PU-2, PG-1,and PG-2) form a six-transistor (6T) SRAM, and the semiconductor devicein FIG. 6A is an 8T-SRAM device. One of ordinary skill in the art,however, should understand that features of the various embodimentsdescribed herein may be used for forming other types of devices, such asa 10T-SRAM device or other integrated circuits.

Reference is made to FIGS. 7A and 7B. A dielectric layer 250 is formedat outer sides of the gate spacers on the substrate 210. The dielectriclayer 250 includes silicon oxide, oxynitride or other suitablematerials. The dielectric layer 250 may include a single layer ormultiple layers. The dielectric layer 250 is formed by a suitabletechnique, such as CVD or ALD. A chemical mechanical planarization (CMP)process may be applied to remove excessive dielectric layer 250 andexpose the top surface of the dummy gates 241, 243, 245, 247, and 249 toa subsequent dummy gate removing process.

A replacement gate (RPG) process scheme is employed. In someembodiments, in a RPG process scheme, a dummy polysilicon gate is formedfirst and is replaced later by a metal gate after high thermal budgetprocesses are performed. In some embodiments, the dummy gates 241, 243,245, 247, and 249 (see FIG. 6A) are removed to form recesses R with thegate spacers and the dielectric layer 250 as its sidewalls. In someembodiments, the dummy gates 241, 243, 245, 247, and 249 are removedwhile the gate dielectric 230 retains. The dummy gates 241, 243, 245,247, and 249 may be removed by dry etch, wet etch, or a combination ofdry and wet etch. For example, a wet etch process may include exposureto a hydroxide containing solution (e.g., ammonium hydroxide), deionizedwater, and/or other suitable etchant solutions.

Reference is made to FIGS. 8A and 8B. A plurality of metal gates 261,263, 265, 267, and 269 are respectively formed in the recesses R. Ingreater detail, a gate dielectric layer 272 is formed on the substrate210 and in the recesses R. The gate dielectric layer 272 may be a high-kdielectric layer such as hafnium oxide (HfO2). Alternatively, the high-kdielectric layer may optionally include other high-k dielectrics, suchas TiO₂, HfZrO, Ta₂O₃, HfSiO₄, ZrO₂, ZrSiO₂, combinations thereof, orother suitable material. The gate dielectric layer 272 may be formed byatomic layer deposition (ALD) and/or other suitable methods. By way ofexample and not limitation, the gate 269 may be referred to as a firstgate structure, the gate 261 may be referred to as a second gatestructure, the gate 265 may be referred to as a third gate structure,the gate 263 may be referred to as a fourth gate structure, and the gate267 may be referred to as a fifth gate structure.

A work function metal layer 274 is then formed on the gate dielectriclayer 272. In some embodiments, the work function metal layer 274 mayinclude a single layer or multi layers, such as a work function film, aliner film, a wetting film, and an adhesion film. The work functionmetal layer 274 may include Ti, TiAl, TiAlN, TaC, TaCN, TaSiN, Mn, Zr,TiN, TaN, Ru, Mo, WN, Co, Al, or any suitable materials. For example,the work function metal layer 274 includes at least one of Ti, Al, orTiAl when the metal gate is part of an N-channel transistor.Alternatively, the work function metal layer 274 includes at least oneof TiN, Co, WN, or TaC when the metal gate is part of a P-channeltransistor. The work function metal layer 274 may be formed by ALD, PVD,CVD, or other suitable process.

The remaining recesses R are filled with a metal layer 276 on the workfunction metal layer 274. In some embodiments, the metal layer 276includes tungsten (W). The metal layer 276 is deposited by ALD, PVD,CVD, or other suitable process. In some other embodiments, the metallayers 276 include aluminum (Al), copper (Cu) or other suitableconductive material. After forming the gate dielectric layer 272, themetal layer 274 and the metal layer 276, a planarization process may beperformed to remove the metal gate layers from the surface of thedielectric layer 250.

In FIG. 8A, the semiconductor fin 212 and the metal gate 261 form apull-down transistor PD-1, and the semiconductor fin 213 and the metalgate 261 form a pull-up transistor PU-1. In other words, the pull-downtransistor PD-1 and the pull-up transistor PU-1 share the metal gate261. The semiconductor fin 215 and the metal gate 263 form a pass-gatetransistor PG-2. The semiconductor fin 218 and the metal gate 265 form aread-pass-gate transistor RPG. The semiconductor fin 212 and the metalgate 267 form another pass-gate transistor PG-1. The semiconductor fin214 and the metal gate 269 form another pull-up transistor PU-2, thesemiconductor fin 215 and the metal gate 269 form another pull-downtransistor PD-2, and the semiconductor fin 218 and the metal gate 269form a read-pull-down transistor RPD. In other words, the pull-uptransistor PU-2, the pull-down transistor PD-2 and the read-pull-downtransistor RPD share the metal gate 269. Therefore, the transistors inthe write portion 210 w (i.e., transistors PD-1, PD-2, PU-1, PU-2, PG-1,and PG-2) form a six-transistor (6T) SRAM, and the semiconductor devicein FIG. 8A is an 8T-SRAM device. One of ordinary skill in the art,however, should understand that features of the various embodimentsdescribed herein may be used for forming other types of devices, such asa 10T-SRAM device or other integrated circuits. By way of example andnot limitation, the transistor PD-2 is referred to as a first write-portpull-down transistor, the transistor PD-1 is referred to as a secondwrite-port pull-down transistor, the transistor PU-1 is referred to as afirst write-port pull-up transistor, the transistor PU-2 is referred toas a second write-port pull-up transistor, the transistor PG-2 isreferred to as a first write-port pass-gate transistor, and thetransistor PG-1 is referred to as a second write-port pass-gatetransistor.

In FIGS. 8A and 8B, the transistors PD-2 and RPD share the same metalgate 269. That is, the gate of the transistor PD-2 and the gate of thetransistor RPD are integrally formed and thus have the substantially thesame configuration. For example, a portion of the work function layer274 disposed on the semiconductor fin 215 of the transistor PD-2 has athickness substantially the same as a thickness of another portion ofthe work function layer 274 disposed on the semiconductor fin 218 of thetransistor RPD. The term “substantially” as used herein may be appliedto modify any quantitative representation which could permissibly varywithout resulting in a change in the basic function to which it isrelated. The threshold voltages of the transistors PD-2 and RPD aretuned by the doping process shown in FIGS. 4A and 4B (and FIGS. 5A and5B). Since the transistor PD-2 includes the channel doping layer 217,the threshold voltage of the transistor PD-2 is raised, such that thethreshold voltage of the transistor PD-2 is higher than the thresholdvoltage of the transistor RPD although they share the same metal gate269. Furthermore, since the transistor RPD is non-doped in FIG. 8B, thechannel of the transistor RPD does not have a doping damage, and thedevice performance of the transistor RPD is improved to allow rapidreading speed.

Moreover, the four transistors PU-1, PU-2, PD-1, and PD-2 form twocross-coupled inverters, such that the threshold voltages of thetransistors PD-1 and PD-2 can be the same in some embodiments. However,since the transistor PD-2 share the metal gate 269 with the transistorRPD, which has lower threshold voltage than the transistor PD-2, thethreshold voltage of the transistor PD-2 may be slightly dropped. Tocompensate this effect, the channel doping layer 217 of the transistorPD-2 has a doping concentration greater than that of the channel dopinglayer 216 of the transistor PD-1. As such, the transistors PD-1 and PD-2can have substantially the same threshold voltage.

FIG. 9 is a top view of a semiconductor device in accordance with someembodiments of the present disclosure. In FIG. 9 , the semiconductordevice is an SRAM device including four memory cells 200 a, 200 b, 200c, and 200 d. In some other embodiments, however, the number of thememory cells 200 a, 200 b, 200 c, and 200 d in the SRAM device is notlimited in this respect. The memory cell 200 a has substantially thesame layout as the semiconductor device in FIG. 8A. In FIG. 9 , when thememory cells 200a-200 d are arranged together to form an array (an SRAMdevice herein), the cell layouts may be flipped or rotated to enablehigher packing densities. Often by flipping the cell over a cellboundary or axis and placing the flipped cell adjacent the originalcell, common nodes and connections can be combined to increase packingdensity. For example, the memory cells 200a-200 d are mirror images andin rotated images of each other. Specifically, the memory cells 200 aand 200 b are mirror images across a Y-axis, as is the memory cells 200c and 200 d. The memory cells 200 a and 200 c are mirror images acrossan X-axis, as is the memory cells 200 b and 200 d. Further, the diagonalmemory cells (the memory cells 200 a and 200 d; the memory cells 200 band 200 c) are rotated images of each other at 180degrees.

FIGS. 10A to 13A are top views of a method for manufacturing asemiconductor device at various stages in accordance with someembodiments of the present disclosure, and FIGS. 10B to 13B arecross-sectional views taken along lines A-A and B-B of FIGS. 10A to 13A.The manufacturing processes of FIGS. 2A to 5B are performed in advance.Since the relevant manufacturing details are similar to FIGS. 2A to 5B,and, therefore, a description in this regard will not be repeatedhereinafter. Reference is made to FIGS. 10A and 10B. The mask 320 ofFIGS. 5A and 5B is removed. In some embodiments, still another mask 330is formed on the substrate 210. The mask 330 covers the write portion210 w and exposes the read portion 210 r. Then, still another channeldoping process is performed to form a channel doping layer 219 insemiconductor fin 218. In some embodiments, N dopants or P dopants suchas arsenic (As) or boron (B) can be doped into the semiconductor fin 218to form the channel doping layer 219. In some embodiments, theimplantation results in a surface layer of the semiconductor fin 218 tobe doped, wherein the surface layer includes the top surface layer andthe opposite sidewall surface layers of the semiconductor fin 218. Theinner portions of the semiconductor fin 218 wrapped by the channeldoping layer 219, however, may be free from the dopants. In alternativeembodiments, the implantation results in an entirety of semiconductorfin 218 to be doped with the dopants. After the process of FIGS. 10A and10B, the doping concentration of the channel doping layer 219 is lowerthan the doping concentration of the channel doping layers 216 and 217.

Reference is made to FIGS. 11A and 11B. A plurality of gate dielectrics230 are formed on the semiconductor fins 212-215 and 218. For clarity,the gate dielectrics 230 are illustrated in FIG. 11B and are omitted inFIG. 11A. The gate dielectrics 230 of FIG. 11B and FIG. 6A may have thesame or different material and/or manufacturing processes, and thepresent disclosure is not limited in this respect. A plurality of dummygates 241, 243, 245, 247, and 249 are then formed on portions of thesemiconductor fins 212-215 and 218 and expose another portions of thesemiconductor fins 212-215 and 218. The dummy gates 241, 243, 245, 247,and 249 of FIG. 11A and FIG. 6A may have the same or different materialand/or manufacturing processes, and the present disclosure is notlimited in this respect.

Reference is made to FIGS. 12A and 12B. A plurality of gate spacers (notshown) are formed over the substrate 210 and along the sides of thedummy gates 241, 243, 245, 247, and 249. A dielectric layer 250 isformed at outer sides of the gate spacers on the substrate 210. Areplacement gate (RPG) process scheme is employed. In some embodiments,the dummy gates 241, 243, 245, 247, and 249 (see FIG. 10A) are removedto form recesses R with the gate spacers and the dielectric layer 250 asits sidewalls.

Reference is made to Figs. 13A and 13B. A plurality of metal gates 261,263, 265, 267, and 269 are respectively formed in the recesses R. Ingreater detail, a gate dielectric layer 272 is formed on the substrate210 and in the recesses R. A work function metal layer 274 is thenformed on the gate dielectric layer 272. The remaining recesses R arefilled with a metal layer 276 on the work function metal layer 274.After forming the gate dielectric layer 272, the metal layer 274 and themetal layer 276, a planarization process may be performed to remove themetal gate layers from the surface of the dielectric layer 250.

In FIG. 13A, the semiconductor fin 212 and the metal gate 261 form apull-down transistor PD-1, and the semiconductor fin 213 and the metalgate 261 form a pull-up transistor PU-1 . In other words, the pull-downtransistor PD-1 and the pull-up transistor PU-1 share the metal gate261. The semiconductor fin 215 and the metal gate 263 form a pass-gatetransistor PG-2. The semiconductor fin 218 and the metal gate 265 form aread-pass-gate transistor RPG. The semiconductor fin 212 and the metalgate 267 form another pass-gate transistor PG-1. The semiconductor fin214 and the metal gate 269 form another pull-up transistor PU-2, thesemiconductor fin 215 and the metal gate 269 form another pull-downtransistor PD-2, and the semiconductor fin 218 and the metal gate 269form a read-pull-down transistor RPD. In other words, the pull-uptransistor PU-2, the pull-down transistor PD-2 and the read-pull-downtransistor RPD share the metal gate 269. Therefore, the transistors inthe write portion 210 w (i.e., transistors PD-1, PD-2, PU-1, PU-2, PG-1,and PG-2) form a six-transistor (6T) SRAM, and the semiconductor devicein FIG. 8A is an 8T-SRAM device. One of ordinary skill in the art,however, should understand that features of the various embodimentsdescribed herein may be used for forming other types of devices, such asa 10T-SRAM device or other integrated circuits.

According to some embodiments, a semiconductor device includes asubstrate, a first transistor, and a second transistor. The firsttransistor is disposed on the substrate. The second transistor isdisposed on the substrate. A gate of the first transistor and a gate ofthe second transistor are integrally formed, and the first transistorand the second transistor have different threshold voltages.

According to some embodiments, a semiconductor device includes asubstrate, a first transistor, and a second transistor. The firsttransistor is disposed on the substrate and includes an active regionand a metal gate. The active region includes a channel doping regiontherein. The metal gate is disposed on the active region of the firsttransistor. The second transistor is disposed on the substrate andadjacent to the first transistor. The second transistor includes anactive region and the metal gate disposed on the active region of thesecond transistor.

According to some embodiments, a method for manufacturing asemiconductor device includes forming a first active region and a secondactive region in a substrate. The first active region is doped. A metalgate is integrally formed on the first active region and the secondactive region. The metal gate and the first active region form a firsttransistor, and the metal gate and the second active region form asecond transistor.

In some embodiments, a static random access memory (SRAM) cell includessubstrate, a first semiconductor fin, a first gate structure, a secondsemiconductor fin, and a second gate structure. The substrate has afirst p-well and an n-well bordering the first p-well. The firstsemiconductor fin extends within the first p-well. The first gatestructure extends across the first semiconductor fin and forms a firstwrite-port pull-down transistor with the first semiconductor fin. Thesecond semiconductor fin extends within the n-well. The second gatestructure extends across the second semiconductor fin and forms a firstwrite-port pull-up transistor with the second semiconductor fin. Achannel region of the first write-port pull-down transistor has a higherdoping concentration than a channel region of the first write-portpull-up transistor.

In some embodiments, a static random access memory (SRAM) cell includesa substrate, a first semiconductor fin, a second semiconductor fin, anda first gate structure. The substrate has a first p-well and an n-welladjacent the first p-well. The first semiconductor fin is over thefirst, p-well. The second semiconductor fin is over the n-well. Thefirst gate structure extends across the first and second semiconductorfins to form a first write-port pull-down transistor with the firstsemiconductor fin and a first write-port pull-up transistor with thesecond semiconductor fin. A channel region of the first write-portpull-down transistor has a higher doping concentration than a channelregion of the first write-port pull-up transistor.

In some embodiments, a static random access memory (SRAM) cell includesfirst and second semiconductor fins and a first gate structure. Thefirst and second semiconductor fins extend along a first direction overa p-well. The first gate structure extends along a second directionperpendicular to the first direction. The first gate structure forms awrite-port pull-down transistor with the first semiconductor fin andforms a read-port pull-down transistor with the second semiconductorfin. The first gate structure has a continuous work function metalspanning at least the write-port pull-down transistor and the read-portpull-down transistor. The work function metal has a thickness in thewrite-port pull-down transistor substantially the same as in theread-port pull-down transistor.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A static random access memory (SRAM) cellcomprising: a substrate having a first p-well and an n-well borderingthe first p-well; a first semiconductor fin extending within the firstp-well; a first gate structure extending across the first semiconductorfin and forming a first write-port pull-down transistor with the firstsemiconductor fin; a second semiconductor fin extending within then-well; a second gate structure extending across the secondsemiconductor fin and forming a first write-port pull up transistor withthe second semiconductor fin, wherein a channel region of the firstwrite-port pull-down transistor has a higher p-type doping concentrationthan a channel region of the first write-port pull up transistor; athird semiconductor fin extending within a second p-well spaced from thefirst p-well by the n-well, wherein the second gate structure extendsacross the third semiconductor fin and forms a second write-portpull-downn type transistor with the third semiconductor fin; a fourthsemiconductor fin extending within the first p-well and forming aread-port pull-down transistor with the first gate structure, wherein ap-type doping concentration of a channel region of the second write-portpull-down transistor is lower than the p-type doping concentration ofthe channel region of the first write-port pull-down transistor andhigher than a p-type doping concentration of a channel region of theread-port pull-down transistor; and a fourth gate structure extendingacross the first semiconductor fin and forming a first write-portpass-gate transistor with the first semiconductor fin, wherein a channelregion of the first write-port pass-gate transistor has a p-type dopingconcentration lower than the p-type doping concentration of the channelregion of the first write-port pull-down transistor.
 2. The SRAM cell ofclaim 1, further comprising: a third gate structure extending across thefourth semiconductor fin and forming a read-port pass-gate transistorwith the fourth semiconductor fin, wherein a channel region of theread-port pass-gate transistor has a doping concentration lower than thep-type doping concentration of the channel region of the firstwrite-port pull-downn type transistor.
 3. The SRAM cell of claim 2,wherein the third gate structure is substantially aligned with thesecond gate structure.
 4. The SRAM cell of claim 2, wherein the thirdgate structure has a length greater than a length of the fourth gatestructure.
 5. The SRAM cell of claim 1, where the fourth gate structureis substantially aligned with the second gate structure.
 6. The SRAMcell of claim 1, further comprising: a fifth gate structure extendingacross the third semiconductor fin and forming a second write-portpass-gate transistor with the third semiconductor fin, wherein a channelregion of the second write-port pass-gate transistor has a dopingconcentration lower than the p-type doping concentration of the channelregion of the first write-port pull-down transistor.
 7. The SRAM cell ofclaim 6, wherein the fifth gate structure is substantially aligned withthe first gate structure.
 8. The SRAM cell of claim 1, wherein the firstgate structure is misaligned with the second gate structure.
 9. The SRAMcell of claim 1, wherein the second write-port pull-down transistor hasa threshold voltage not lower than a threshold voltage of the firstwrite-port pull-down transistor.
 10. A static random access memory(SRAM) cell comprising: a substrate having a first p-well, a secondp-well, and an n-well interposing the first p- well and the secondp-well; a first semiconductor fin over the first p-well; a fifthsemiconductor fin over the n-well; a first gate structure extendingacross the first and fifth semiconductor fins to form a first write-portpull-down transistor with the first semiconductor fin and a secondwrite- port pull-up transistor with the fifth semiconductor fin, whereina channel region of the first write-port pull-down transistor has ap-type doping concentration higher than a doping concentration of achannel region of the second write-port pull-up transistor; a secondwrite-port pull-down transistor over the second p-well, wherein achannel region of the second write-port pull-down transistor has ap-type doping concentration lower than the p-type doping concentrationof the channel region of the first write-port pull- down transistor; afourth semiconductor fin spaced from the fifth semiconductor fin in partby the first semiconductor fin, wherein the first gate structure extendsacross the fourth semiconductor fin and forms a read-port pull-downtransistor with the fourth semiconductor fin, and a channel region ofthe read-port pull-down transistor has a p-type doping concentrationlower than the p-type doping concentration of the channel region of thesecond write-port pull-down transistor; and a fourth gate structureextending across the first semiconductor fin and forming a firstwrite-port pass-gate transistor with the first semiconductor fin,wherein a channel region of the first write-port pass-gate transistorhas a p-type doping concentration lower than the p-type dopingconcentration of the channel region of the first write-port pull-downtransistor.
 11. The SRAM cell of claim 10, further comprising: a thirdgate structure extending across the fourth semiconductor fin and forminga read-port pass-gate transistor with the fourth semiconductor fin,wherein a channel region of the read-port pass-gate transistor has adoping concentration lower than the p-type doping concentration of thechannel region of the first write-port pull-down transistor.
 12. TheSRAM cell of claim 11, wherein the third gate structure has a lengthgreater than a length of the fourth gate structure.
 13. The SRAM cell ofclaim 10, further comprising: a second write-port pass-gate transistorover the second p-well spaced from the first p-well by the n-well,wherein a channel region of the second write-port pass-gate transistorhas a doping concentration lower than the p-type doping concentration ofthe channel region of the first write-port pull-down transistor.
 14. TheSRAM cell of claim 10, wherein the second write-port pull-downtransistor has a threshold voltage not lower than a threshold voltage ofthe first write-port pull-down transistor.
 15. A static random accessmemory (SRAM) cell comprising: a first semiconductor fin and a fourthsemiconductor fin extending along a first direction over a first p-well;a first gate structure extending along a second direction perpendicularto the first direction, the first gate structure forming a firstwrite-port pull-down transistor with the first semiconductor fin and aread-port pull-down transistor with the fourth semiconductor fin,wherein the first gate structure has a continuous work function metalspanning the first write-port pull-down transistor and the read-portpull-down transistor; a third semiconductor fin extending along thefirst direction over a second p-well spaced from the first p-well; asecond gate structure extending along the second direction and forming asecond write-port pull-down transistor with the third semiconductor fin,wherein a channel region of the second write-port pull-down transistorhas a p-type doping concentration lower than a p-type dopingconcentration of a channel region of the first write-port pull-downtransistor and higher than a p-type doping concentration of a channelregion of the read-port pull-down transistor; and a fourth gatestructure forming a first write-port pass-gate transistor with the firstsemiconductor fin, wherein a channel region of the first write-portpass-gate transistor has a p-type doping concentration lower than thep-type doping concentration of the channel region of the firstwrite-port pull-down transistor.
 16. The SRAM cell of claim 15, whereina threshold voltage of the first write-port pull-down transistor ishigher than a threshold voltage of the read-port pull-down transistor.17. The SRAM cell of claim 16, wherein a metal composition of the workfunction metal within the first write-port pull-down transistor is sameas a metal composition of the work function metal within the read-portpull-downsecond n type transistor.
 18. The SRAM cell of claim 16,wherein a material composition of the first gate structure within thefirst write-port pull-downn type transistor is same as a materialcomposition of the first gate structure within the read-port pull-downtransistor.
 19. The SRAM cell of claim 15, wherein the second write-portpull-down transistor has a threshold voltage not lower than a thresholdvoltage of the first write-port pull-down transistor.
 20. The SRAM cellof claim 15, further comprising: a read-port pass-gate transistor,wherein a gate structure of the read-port pass-gate transistor has alength greater than a length of the fourth gate structure.